Battery

ABSTRACT

This application relates to a battery comprising a positive electrode plate, a separator, and a negative electrode plate, wherein the positive electrode plate comprises a positive electrode current collector and at least two layers of positive active material coated on at least one surface of the positive electrode current collector, and wherein an underlying positive active material layer in contact with the positive electrode current collector comprises a first positive active material, a first polymer material and a first conductive material; and wherein an upper positive active material layer in contact with the underlying positive active material layer and away from the positive electrode current collector comprises a second positive active material, a second polymer material and a second conductive material, and the first polymer material comprises fluorinated polyolefin and/or chlorinated polyolefin polymer material. The battery has good safety and improved electrical properties.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on and claims priority to Chinese PatentApplication No. 201811366715.3 filed on Nov. 16, 2018, which isincorporated herein by reference in its entirety.

TECHNICAL FIELD

This application belongs the field of electrochemical technology, andmore particularly, this application relates to a battery.

BACKGROUND

Lithium-ion batteries are widely used in electric vehicles and consumerelectronics because of their high energy density, high output power,long cycle life and small environmental pollution. However, lithium-ionbatteries are prone to fire and explode when subjected to abnormalconditions such as crushing, bumping or puncture, causing serious harm.Therefore, the safety problem of lithium-ion batteries greatly limitsthe application and popularity of lithium-ion batteries.

Although researchers have proposed many ways to improve the safety ofbatteries, there is still no very effective means for the safety hazardcaused by the puncture of the battery. In view of this, it is indeednecessary to provide a battery that has improved battery safety,especially nail penetration safety.

SUMMARY

It is an object of this application to provide a battery with improvedsafety, especially with improved nail penetration safety.

It is a further object of this application to provide a battery havingexcellent properties such as good safety, improved electricalproperties, and good processibility.

This application provides a battery comprising a positive electrodeplate, a separator, and a negative electrode plate, wherein the positiveelectrode plate comprises a positive electrode current collector and atleast two layers of positive active material coated on at least onesurface of the positive electrode current collector, and wherein anunderlying positive active material layer in contact with the positiveelectrode current collector comprises a first positive active material,a first polymer material and a first conductive material, and based onthe total weight of the underlying positive active material layer, thefirst positive active material has a content of A % by weight, the firstpolymer material has a content of B % by weight, and the firstconductive material has a content of C % by weight; and wherein an upperpositive active material layer in contact with the underlying positiveactive material layer and away from the positive electrode currentcollector comprises a second positive active material, a second polymermaterial and a second conductive material, and based on the total weightof the upper positive active material layer, the second positive activematerial has a content of A′ % by weight, the second polymer materialhas a content of B′ % by weight, and the second conductive material hasa content of C′ % by weight, wherein A %<A′ %, B %>B′ %, C %≥C′ %, andthe first polymer material comprises fluorinated polyolefin and/orchlorinated polyolefin polymer material.

The battery of this application has good safety and improved electricalproperties.

DESCRIPTION OF THE DRAWINGS

The battery and the beneficial effects of this application will bedescribed in detail below with reference to the accompanying drawingsand specific embodiments.

FIG. 1 is a schematic structural view of a positive electrode plateaccording to an embodiment of this application, in which 10—a positiveelectrode current collector; 14—an upper positive active material layer;12—an underlying positive active material layer.

DETAILED DESCRIPTION

A large number of experimental results show that internal short circuitof battery is the root cause of the safety hazard of lithium-ionbatteries. The root cause of internal short circuit of battery is theelectrical connection between the positive electrode plate and thenegative electrode plate inside the battery. In the abnormal case suchas nail penetration, the direct contact of the metal burr (usually Almetal burr) produced in the positive electrode plate with the negativeelectrode plate can cause internal short circuit of battery. Theinventors of this application have found that the metal burr of thepositive electrode plate can be effectively masked (or wrapped) by thecoating design of the positive electrode plate, thereby preventinginternal short circuit of battery and the resulting thermal runaway ofthe battery.

This application discloses a battery comprising a positive electrodeplate, a separator and a negative electrode plate. The positiveelectrode plate in the battery comprises a positive electrode currentcollector and at least two layers of positive active material coated onat least one surface of the positive electrode current collector. Sincethe at least two layers of the positive active material are respectivelyformed on the current collector and they are usually tightly bondedtogether, when the coating is peeled off from the current collector,generally a coating as a whole is obtained. Therefore, the at least twolayers of the positive active material are collectively referred to as apositive electrode film layer.

The underlying positive active material layer in contact with thepositive electrode current collector comprises a first positive activematerial, a first polymer material and a first conductive material, andbased on the total weight of the underlying positive active materiallayer, the first positive active material has a content of A % byweight, the first polymer material has a content of B % by weight, andthe first conductive material has a content of C % by weight.

The upper positive active material layer in contact with the underlyingpositive active material layer and away from the positive electrodecurrent collector comprises a second positive active material, a secondpolymer material and a second conductive material, and based on thetotal weight of the upper positive active material layer, the secondpositive active material has a content of A′ % by weight, the secondpolymer material has a content of B′ % by weight, and the secondconductive material has a content of C′ % by weight,

wherein A %<A′ %, B %>B′ %, C %≥C′ %, and

wherein the first polymer material comprises fluorinated polyolefinand/or chlorinated polyolefin polymer material.

That is to say, the content of the polymer material and the content ofthe conductive material in the underlying positive active material layerare both higher than those of the upper positive active material layer.Since the underlying positive active material layer contains arelatively high content of the first polymer material, the underlyingpositive active material layer has the property of a binder layercompared with the upper positive active material layer, so that underabnormal conditions such as nail penetration the underlying positiveactive material layer can wrap the metal burrs that may be generated inthe current collector to effectively prevent internal short circuit ofbattery.

Preferably, the adhesion force between the positive electrode film layerand the positive electrode current collector is 10 N/m or more. If theadhesion force is insufficient, the underlying positive active materiallayer may not effectively wrap metal burrs that may be generated in thecurrent collector.

Further, it is found that when the content of the first polymer materialin the underlying positive active material layer is relatively high, thefirst polymer material acts with the first conductive material together,so that the underlying positive active material layer has a positivetemperature coefficient effect (i.e., a PTC effect). The PTC actionprinciple of the underlying positive active material layer is that: atnormal temperature, the underlying positive active material layerconducts the electrons by virtue of a good conductive network formedbetween the first conductive materials; when the temperature increases,the volume of the first polymer material begins to expand, the spacingbetween the particles of the first conductive material increases, thusthe conductive network is partially blocked, and the resistance of theunderlying positive active material layer gradually increases; when acertain temperature (for example, the operating temperature) is reached,the conductive network is almost completely blocked, then the currentapproaches zero. The DC (direct current) resistance growth rate is acommon parameter for characterizing the PTC effect. According to apreferred embodiment of this application, the underlying positive activematerial layer exhibits a PTC effect, and preferably, the battery has aDC resistance growth rate of 100% or more at 130° C.

Therefore, the underlying positive active material layer is disposedbetween the current collector and the upper positive active materiallayer as a primer layer, and simultaneously exerts the technical effectsof a binder layer and a PTC safety coating, thereby greatly improvingthe nail penetration safety of battery.

In addition, the underlying positive active material layer furthercomprises a first positive active material, which can stabilize andimprove the technical effect of the underlying positive active materiallayer as a binder layer and a PTC safety coating from the followingthree aspects:

(1) hindering the adverse effects of a solvent (such as NMP or the like)in the upper positive active material layer or an electrolyte on thefirst polymer material (for example, fluorinated polyolefin and/orchlorinated polyolefin polymer material) in the underlying positiveactive material layer, such as dissolving or swelling and the like; (2)beneficially ensuring that the underlying positive active material layeris not easily deformed during compaction process of electrode plate toavoid direct contact between the current collector and the upperpositive active material layer; (3) improving the response speed and thelike of the PTC effect of the underlying positive active material layer.

In a preferred embodiment of this application, the positive activematerial layer is a two-layer structure, that is to say, consisting ofthe upper positive active material layer and the underlying positiveactive material layer. This facilitates the simplification of themanufacturing process.

FIG. 1 shows a schematic structural view of a positive electrode plateaccording to some embodiments of this application, wherein 10 representsa current collector, 14 represents an upper positive active materiallayer, and 12 represents an underlying positive active material layer.

It is easy to understand that FIG. 1 only shows the embodiment in whichthe positive active material layer is only provided on one side of thepositive electrode collector 10, the underlying positive active materiallayer 12 and the upper positive active material layer 14 may berespectively disposed on both sides of the positive current collector 10in other embodiments.

Preferably, at least two layers of the positive active material aredisposed on both surfaces of the positive electrode current collector tomore effectively improve the nail penetration safety problem of theconventional lithium-ion battery.

In order to improve the nail penetration safety problem of theconventional lithium-ion battery, this application proposes a number oftechnical improvements, and combines various technical means to improvethe nail penetration safety problem of the lithium-ion battery.

The specific composition and structure of the positive electrode platein the battery of this application will be described in more detailbelow.

(1) Underlying Positive Active Material Layer

First, the inventors have found that the stability of the underlyingpositive active material layer and the properties thereof as a PTC layerand a binder layer can be improved by selecting the first polymermaterial in the underlying positive active material layer.

In this application, the fluorinated polyolefin and/or chlorinatedpolyolefin material actually functions in two ways, i.e. both as a PTCmatrix and as a binder.

The underlying positive active material layer composed of thefluorinated polyolefin and/or chlorinated polyolefin material and thefirst conductive material may function as a PTC thermistor layer, andthe operating temperature range may suitably be 80° C. to 160° C. Thusit can improve the high temperature safety performance of the battery.

The fluorinated polyolefin and/or chlorinated polyolefin as the firstpolymer material of the underlying positive active material layer serveas both a PTC matrix and a binder, thereby facilitating the preparationof a thinner underlying positive active material layer. Moreover, it isadvantageous to improve the adhesion of the underlying positive activematerial layer and the adhesion force with the current collector.

In this application, the fluorinated polyolefin and/or chlorinatedpolyolefin refers to polyvinylidene fluoride (PVDF), polyvinylidenechloride (PVDC), modified PVDF, or modified PVDC. For example, thefluorinated polyolefin and/or chlorinated polyolefin may be selectedfrom PVDF, carboxylic acid modified PVDF, acrylic acid modified PVDF,PVDF copolymer, PVDC, carboxylic acid modified PVDC, acrylic acidmodified PVDC, PVDC copolymer, or any mixture thereof.

In this application, based on the total weight of the underlyingpositive active material layer, the content of the first polymermaterial B % generally satisfies 35 wt %≤B %≤75 wt %.

Since the content of the first polymer material in the underlyingpositive active material layer is higher than the content of the secondpolymer material in the upper positive active material layer and isusually as high as 35 wt % or more, the underlying positive activematerial layer has the property of a binder layer compared with theupper positive active material layer, so that under abnormal conditionssuch as nail penetration the underlying positive active material layercan wrap the metal burrs that may be generated in the current collectorto effectively prevent internal short circuit of battery. The content ofthe first polymer material is preferably from 40% by weight to 75% byweight, more preferably from 50% by weight to 75% by weight.

The first polymer material may all be a fluorinated polyolefin and/orchlorinated polyolefin polymer material.

If all the first polymer material in the underlying positive activematerial layer is a fluorinated polyolefin and/or chlorinatedpolyolefin, the following technical problems may occur:

(1) Since the fluorinated polyolefin and/or chlorinated polyolefin havea large dissolution and swelling in an organic oil solvent (such as NMPor the like) and an electrolyte, during the process of coating the upperpositive active material layer, if the coating speed is too fast, it iseasy to cause cracking of the upper positive active material layer dueto uneven stress;

(2) Since the fluorinated polyolefin and/or chlorinated polyolefin havea large dissolution and swelling in an organic oil solvent (such as NMPor the like) and an electrolyte, the introduction of the underlyingpositive active material layer causes a large increase of battery DCR(DC internal resistance), which is not conducive to the improvement ofthe dynamic performance of the battery.

Since in the oil solvent the solubility of the difficultly solublepolymer material is smaller than the solubility of the fluorinatedpolyolefin and/or chlorinated polyolefin, the above technical problemscan be solved by incorporating the difficultly soluble polymer materialinto the first polymer material of the underlying positive activematerial layer. That is to say, the difficultly soluble polymer materialacts as a “difficultly soluble component” to hinder the too largedissolution and swelling of the fluorinated polyolefin and/orchlorinated polyolefin of the first polymer material in an organic oilsolvent (such as NMP) and an electrolyte, as such to solve the problemof cracking and excessive DCR growth.

Therefore, as an improvement of this application, the first polymermaterial may also be a mixed material of a fluorinated polyolefin and/orchlorinated polyolefin polymer material with other difficultly solublepolymer materials, wherein in an oil solvent (preferably, in NMP) thesolubility of the difficultly soluble polymer material is smaller thanthe solubility of the fluorinated polyolefin and/or chlorinatedpolyolefin polymer material.

The difficultly soluble polymer material may be one of anoil-dispersible polymer material or a water-dispersible polymermaterial.

When the difficultly soluble polymer material is an oil-dispersiblepolymer material, the oil-dispersible polymer material is preferablyselected from at least one of oil-dispersible polyacrylonitrile,oil-dispersible polyacrylic acid, oil-dispersible polyacrylate,oil-dispersible polyacrylic acid-acrylate, oil-dispersiblepolyacrylonitrile-acrylic acid and oil-dispersiblepolyacrylonitrile-acrylate.

When the difficultly soluble polymer material is a water-dispersiblepolymer material, the water-dispersible polymer material is preferablyselected from at least one of water-dispersible polyacrylic acid,water-dispersible polyurethane, water-dispersible polyvinyl alcohol,water-dispersible PVDF, water-dispersible polyacrylate,water-dispersible polytetrafluoroethylene, and water-dispersiblepolyacrylonitrile.

In this application, the water-dispersible polymer material means thatthe polymer molecular chain is completely extended and dispersed inwater, and the oil-dispersible polymer material means that the polymermolecular chain is completely extended and dispersed in the oil solvent.Those skilled in the art understand that by using a suitable surfactant,the same type of polymer material can be dispersed in water and oil,respectively. That is to say, by using a suitable surfactant, the sametype of polymer material can be made into a water-dispersible polymermaterial or an oil-dispersible polymer material, respectively. Forexample, those skilled in the art can appropriately selectwater-dispersible polyacrylonitrile or oil-dispersible polyacrylonitrileas the difficultly soluble polymer material in the first polymermaterial, or select water-dispersible polyacrylate or oil-dispersiblepolyacrylate as the difficultly soluble polymer material in the firstpolymer material.

If the solubility of the fluorinated polyolefin and/or chlorinatedpolyolefin polymer material such as PVDF or PVDC in NMP is 100%, thesolubility of the preferred difficultly soluble polymer material in NMPis substantially no more than 30%. For example, the solubility ofoil-dispersible polyacrylonitrile in NMP is about 8%, and that ofoil-dispersible polyacrylate in NMP is 15%; the solubility ofwater-dispersible polymer materials such as water-dispersiblepolyacrylic acid, water-dispersible polyurethane, and water-dispersiblepolyvinyl alcohol in NMP is no more than 5%.

Since the addition of the water-dispersible polymer material as adifficultly soluble polymer material may increase the brittleness of thecoating, which is disadvantageous to the improvement of the safetyperformance of the battery and to the improvement of the cycle life, itis preferred to add the oil-dispersible polymer material as adifficultly soluble polymer material.

When the first polymer material is a mixed material of a fluorinatedpolyolefin and/or chlorinated polyolefin polymer material with adifficultly soluble polymer material (preferably an oil-dispersiblepolymer material), based on the total weight of the underlying positiveactive material layer, the fluorinated polyolefin and/or chlorinatedpolyolefin polymer material generally has a content of B1% satisfying B1%≥17.5wt %, preferably B 1% ≥20wt %, more preferably B1%≥25wt % (Thecontent of the difficultly soluble polymer material can be easilydetermined from B %-B1%). It has been found that when the content B1% ofthe fluorinated polyolefin and/or chlorinated polyolefin polymermaterial is within the above preferred range, the underlying positiveactive material layer exerts a better effect on improving the nailpenetration safety of the battery.

As a modification of another aspect of this application, the fluorinatedpolyolefin and/or chlorinated polyolefin polymer material in the firstpolymer material contained in the underlying positive active materiallayer is preferably subjected to crosslinking treatment. That is to say,the fluorinated polyolefin and/or chlorinated polyolefin polymermaterial is preferably a fluorinated polyolefin and/or chlorinatedpolyolefin having a crosslinked structure.

The crosslinking treatment may be more advantageous for hindering theadverse effects of a solvent (such as NMP or the like) in the upperpositive active material layer or an electrolyte on the fluorinatedpolyolefin and/or chlorinated polyolefin polymer material in the firstpolymer material contained in the underlying positive active materiallayer, such as dissolving or swelling and the like, and for preventingcracking of the upper positive active material layer and the problem ofbattery DCR growth.

The procedure of the crosslinking treatment is known in the art. Forexample, for a fluorinated polyolefin and/or chlorinated polyolefinpolymer matrix, the crosslinking treatment can be achieved byintroducing an activator and a crosslinking agent. The function of theactivator is to remove the HF or HCl from the fluorinated polyolefinand/or chlorinated polyolefin to form a C═C double bond; thecrosslinking agent acts to crosslink the C═C double bond. As anactivator, a strong base-weak acid salt such as sodium silicate orpotassium silicate can be used. The weight ratio of the activator to thepolymer matrix is usually from 0.5% to 5%. The crosslinking agent may beselected from at least one of polyisocyanates (JQ-1, JQ-1E, JQ-2E,JQ-3E, JQ-4, JQ-5, JQ-6, PAPI, emulsifiable MDI, tetraisocyanate),polyamines (propylenediamine, MOCA), polyols (polyethylene glycol,polypropylene glycol, trimethylolpropane), glycidyl ethers(polypropylene glycol glycidyl ether), inorganic substances (zinc oxide,aluminum chloride, aluminum sulfate, sulfur, boric acid, borax, chromiumnitrate), organic substances (styrene, a-methyl styrene, acrylonitrile,acrylic acid, methacrylic acid, glyoxal, aziridine), organosilicons(tetraethyl orthosilicate, tetramethyl orthosilicate, trimethoxysilane),benzenesulfonic acids (p-toluenesulfonic acid, p-toluenesulfonylchloride), acrylates (1,4-butylene glycol diacrylate, ethylene glycoldimethacrylate, TAC, butyl acrylate, HEA, HPA, HEMA, HPMA, MMA), organicperoxides (dicumyl peroxide, bis(2,4-dichlorobenzoyl) peroxide), andmetal organic compounds (aluminum isopropoxide, zinc acetate, titaniumacetylacetonate).

The weight ratio of the crosslinking agent to the fluorinated polyolefinand/or chlorinated polyolefin is from 0.01% to 5%. If too littlecrosslinking agent is used, the crosslinking degree of the fluorinatedpolyolefin and/or chlorinated polyolefin is low, and the cracking cannotbe completely eliminated. If excessive crosslinking agent is used, it iseasy to cause gel during stirring. The activator and the crosslinkingagent may be added after the stirring of the slurry for preparing theunderlying positive active material layer is completed, then performingthe crosslinking reaction, the mixture is uniformly stirred and thencoated to prepare an underlying positive active material layer.

Next, the inventors have found that the first positive active materialin the underlying positive active material layer can function as aninorganic filler to stabilize the underlying positive active materiallayer.

It has been found that the first positive active material contained inthe underlying positive active material layer can function to stabilizeand improve the technical effect of the underlying positive activematerial layer as a binder layer and a PTC safety coating from thefollowing three aspects:

(1) hindering the adverse effects of a solvent (such as NMP or the like)in the upper positive active material layer or an electrolyte on thefluorinated polyolefin and/or chlorinated polyolefin polymer material inthe underlying positive active material layer, such as dissolving orswelling and the like; (2) beneficially ensuring that the underlyingpositive active material layer is not easily deformed during compactionprocess of electrode plate to avoid direct contact between the currentcollector and the upper positive active material layer; (3) improvingthe response speed and the like of the PTC effect of the underlyingpositive active material layer.

The first positive active material contained in the underlying positiveactive material layer corresponds to a barrier substance, therebyfacilitating elimination of the adverse effects of a solvent (such asNMP or the like) in the upper positive active material layer or anelectrolyte on the fluorinated polyolefin and/or chlorinated polyolefinpolymer material contained in the underlying positive active materiallayer, such as dissolution and swelling and the like, and facilitatingthe stabilization of the underlying positive active material layer.

It has also been found that the presence of the first positive activematerial contained in the underlying positive active material layer isalso advantageous for ensuring that the underlying positive activematerial layer is not easily deformed during the electrode platecompaction process. Therefore, it can be well ensured that theunderlying positive active material layer is stably disposed between thecurrent collector and the upper positive active material layer, and thatthe current collector is prevented from directly contacting with theupper positive active material layer, thereby improving the safetyperformance of the battery.

The inventors have also unexpectedly found that the presence of thefirst positive active material contained in the underlying positiveactive material layer can also improve the PTC response speed and thelike of the underlying positive active material layer. The PTC actionprinciple of the underlying positive active material layer is that: atnormal temperature, the underlying positive active material layerconducts the electrons by virtue of a good conductive network formedbetween the first conductive materials; when the temperature increases,the volume of the first polymer material begins to expand, the spacingbetween the particles of the first conductive material increases, thusthe conductive network is partially blocked, and the resistance of theunderlying positive active material layer gradually increases; when acertain temperature (for example, the operating temperature) is reached,the conductive network is almost completely blocked, then the currentapproaches zero. However, in general, when a dynamic equilibrium isreached inside the underlying positive active material layer, theconductive network is partially recovered, so that after reaching acertain temperature (for example, the operating temperature), theresistance of the underlying positive active material layer is not largeas expected, and still very small currents pass. The inventors havefound that, in the presence of the first positive active materialcontained in the underlying positive active material layer, after thevolume expansion of the first polymer material, the first positiveactive material and the first polymer material having an increasedvolume can act to block the conductive network. Therefore, in theoperating temperature range, the underlying positive active materiallayer can better produce the PTC effect. That is to say, the resistancevalue increases faster at a high temperature, and thus the PTC responsespeed is faster. Thereby, the safety performance of the battery can bebetter improved.

Based on the total weight of the underlying positive active materiallayer, the content A % of the first positive active material generallysatisfies 10 wt %≤A %≤60 wt %. If the content is too small, it is notenough to stabilize the underlying positive active material layer; ifthe content is too large, it will affect the PTC properties of theunderlying positive active material layer. The content of the firstpositive active material is preferably from 15% by weight to 45% byweight.

The inventors have found that in addition to the first positive activematerial many other inorganic fillers can also have similar technicaleffects, for example, at least one of a metal oxide, a non-metal oxide,a metal carbide, a non-metal carbide, and an inorganic salt, or at leastone of a conductive carbon coating modified above material, a conductivemetal coating modified above material, or a conductive polymer coatingmodified above materials.

For example, the other inorganic filler may be selected from at leastone of magnesium oxide, aluminum oxide, titanium dioxide, zirconiumoxide, silicon dioxide, silicon carbide, boron carbide, calciumcarbonate, aluminum silicate, calcium silicate, potassium titanate, andbarium sulfate.

However, the inventors have found that using a positive electrodeelectrochemically active material (i.e., a first positive activematerial) or a conductive carbon coating modified, a conductive metalcoating modified or a conductive polymer coating modified positiveelectrode electrochemically active material to replace other inorganicfillers has particular advantages. In this case, the first positiveactive material can also play the roles from the following two aspects:(1) to improve the overcharge performance of the battery. In the PTCsafety coating system composed of the first polymer material and thefirst conductive material, due to that the electrochemically activematerial (i.e., a first positive active material) has thecharacteristics of lithium ion intercalation, the electrochemicallyactive material can be used as an “active site” that participates in theconductive network at the normal operating temperature of the batteryand thus the number of “active site” in the safety coating is increased.In the process of overcharging, the electrochemically active materialwill delithiate, and the delithiation process is becoming more and moredifficult, and the impedance is increasing. Therefore, when the currentpasses, the heat-generating power increases, and the temperature of theprimer layer increases faster, so the PTC effect responds faster, whichin turn can generate PTC effects before the overcharge safety problem ofbattery occurs. Thus the battery overcharge safety performance may beimproved; (2) to contribute charge and discharge capacity. Since theelectrochemically active material (i.e., a first positive activematerial) can contribute a certain charge and discharge capacity at thenormal operating temperature of the battery, the effect of the safetycoating on the electrochemical performance such as capacity of thebattery at the normal operating temperature can be dropped to thelowest.

The first positive active material may be selected from at least one oflithium cobalt oxide, lithium nickel manganese cobalt oxide, lithiumnickel manganese aluminum oxide, lithium iron phosphate, lithiumvanadium phosphate, lithium cobalt phosphate, lithium manganesephosphate, lithium manganese iron phosphate, lithium iron silicate,lithium vanadium silicate, lithium cobalt silicate, lithium manganesesilicate, spinel lithium manganese oxide, spinel lithium nickelmanganese oxide, lithium titanium oxide, or at least one of a conductivecarbon coating modified above material, a conductive metal coatingmodified above material or a conductive polymer coating modified abovematerial. The first positive active material is preferably at least oneof lithium iron phosphate, lithium vanadium phosphate, lithium cobaltphosphate, lithium manganese phosphate, lithium manganese iron phosphateor at least one of a conductive carbon coating modified above material,a conductive metal coating modified above material or a conductivepolymer coating modified above material. The reasons are as follows: (1)lithium iron phosphate, lithium vanadium phosphate, lithium cobaltphosphate, lithium manganese phosphate, lithium manganese iron phosphateand the like have high safety performance, and do not release oxygenwhen overcharged; (2) compared with lithium cobalt oxide, lithium nickelmanganese cobalt oxide and the like, the resistance of the abovematerials increases more when overcharged, so that the undercoatinglayer (i.e., the underlying positive active material layer) generatesmore heat, thus the underlying positive active material layer “executes”PTC effect more quickly.

When the particle size of the first positive active material is toosmall, the specific surface area increases, and the side reactionincreases; when the particle size is too large, the coating thickness ofthe underlying positive active material layer is too large and thethickness is uneven. Preferably, the average particle size D of thefirst positive active material in the underlying positive activematerial layer satisfies 100 nm<D<10 μm, and more preferably 1 μm≤D 6μm. When the particle size of the first positive active material is inthe above range, the effect of blocking the conductive network at a hightemperature can be improved, thereby improving the response speed of theunderlying positive active material layer.

Still preferably, the first positive active material in the underlyingpositive active material layer has a specific surface area (BET) of nomore than 500 m²/g. When the specific surface area of the first positiveactive material increases, the side reaction may increase to affect thebattery performance; and when the specific surface area of the firstpositive active material is too large, a higher proportion of binder isconsumed, which may result in a decrease in the adhesion force betweenthe underlying positive active material layer and the current collectorand the upper positive active material layer, and the internalresistance growth rate is higher. When the specific surface area (BET)of the first positive active material is no more than 500 m²/g, a betteroverall effect can be provided.

Third, the inventors have found that controlling the content of thefirst conductive material in the underlying positive active materiallayer contributes to further optimizing the safety performance of theunderlying positive active material layer.

In addition to the first polymer material and the first positive activematerial, the underlying positive active material layer disposed betweenthe current collector and the upper positive active material layerfurther comprises a first conductive material.

In this application, the content C % of the first conductive materialgenerally satisfies 5 wt %≤C %≤25 wt %, preferably 5% wt %≤C %≤20 wt %,relative to the total weight of the underlying positive active materiallayer.

In addition, the weight ratio of the first conductive material to thefirst polymer material also has an effect on the correct functioning ofthe PTC layer. It has been found that, in the underlying positive activematerial layer, when all of the first polymer material is a fluorinatedpolyolefin and/or chlorinated polyolefin, it is preferred that theweight ratio of the first polymer material to the first conductivematerial is 2 or more, and the PTC effect is better at this time. Whenthe first polymer material is a mixture of a fluorinated polyolefinand/or chlorinated polyolefin with an oil-dispersible polymer material(a difficultly soluble polymer material), preferably the weight ratio ofthe sum of the fluorinated polyolefin and/or chlorinated polyolefin andthe oil-dispersible polymer material to the first conductive material is2 or more, and the PTC effect is better at this time. When the firstpolymer material is a mixture of a fluorinated polyolefin and/orchlorinated polyolefin with a water-dispersible polymer material (adifficultly soluble polymer material), preferably the weight ratio ofthe sum of the fluorinated polyolefin and/or chlorinated polyolefin andthe water-dispersible polymer material to the first conductive materialis 2 or more.

If not satisfying the above weight ratio, the content of the firstconductive material is relatively high, the conductive network may notbe sufficiently disconnected when the temperature increases, therebyaffecting the PTC effect. Moreover, within the above weight ratio, thenail penetration safety can be further improved.

If the weight ratio of the first polymer material to the firstconductive material is too high, the content of the first conductivematerial will be relatively low, which may cause a large increase in theDCR of the battery during normal operation.

More preferably, the above weight ratio is 3 or more and 8 or less.

The first conductive material may be selected from at least one of aconductive carbon-based material, a conductive metal material, and aconductive polymer material, wherein the conductive carbon-basedmaterial is selected from at least one of conductive carbon black,acetylene black, graphite, graphene, carbon nanotubes and carbonnanofibers; the conductive metal material is selected from at least oneof Al powder, Ni powder, and gold powder; and the conductive polymermaterial is selected from at least one of conductive polythiophene,conductive polypyrrole, and conductive polyaniline. The first conductivematerials may be used alone or in combination of two or more

The first conductive materials are typically used in the form of powdersor granules. The particle size may be 5 nm to 500 nm, for example, 10 nmto 300 nm, 15 nm to 200 nm, 15 nm to 100 nm, 20 nm to 400 nm, 20 nm to150 nm, or the like, depending on the specific application environment.

In addition to the first polymer material, the first conductivematerial, and the first positive active material, the underlyingpositive active material layer of this application may also compriseother materials or components, such as binder for promoting the adhesionbetween the underlying positive active material layer and the substrateused as the current collector, and the like. Those skilled in the artcan select other additives according to actual needs. For example, inother embodiments of this application, the underlying positive activematerial layer may further comprise other binders.

In order to simplify the process and save cost, in a preferredembodiment of this application, the underlying positive active materiallayer is substantially free of other binders (“substantially free” meansthat the content is ≤3%, ≤1%, or ≤0.5%).

Furthermore, in some preferred embodiments of this application, theunderlying positive active material layer of this application mayconsist essentially of the first polymer material, the first conductivematerial, and the first positive active material, i.e., it does notcontain a significant amount (e.g., a content of ≤3%, ≤1%, or ≤0.5%) ofother components.

The thickness H of the underlying positive active material layer can beappropriately determined according to actual needs, but preferablysatisfies 1 μm≤H≤20 μm, more preferably 3 μm≤H≤10 μm.

(2) Upper Positive Active Material Layer

The upper positive active material layer of the positive electrode plateaccording to this application may be a commonly used positive activematerial layer for a lithium-ion battery, and also comprises a positiveactive material (a second positive active material), a binder (a secondpolymer material) and a conductive material (a second conductivematerial). The composition of the upper active material layer is thesame as that of the conventional positive active material layer for thepositive electrode plate in the prior art, and its constitution andpreparation method are also well known in the art. However, thisapplication has a limit on the content of each component in the upperpositive active material layer. Based on the total weight of the upperpositive active material layer, the content of the second positiveactive material is A′ % by weight, the content of the second polymermaterial is B′ % by weight, and the content of the second conductivematerial is C′ % by weight, wherein it is necessary to satisfy: A %<A′%, B %>B′ %, C %≥C′ %.

A person skilled in the art can reasonably determine the range of A′ %,B′ %, C′ % according to A %, B %, C % of the underlying positive activematerial layer. Their range may be, for example, as follows: the secondpositive active material satisfies 90 wt %≤5 A′ %≤99 wt %, the secondpolymer material satisfies 0.5 wt %≤B′ %≤5 wt %, and the secondconductive material satisfies 0.5 wt %≤C′ %≤5 wt %.

In this application, the types of the polymer material, the positiveactive material, and the conductive material used in the underlyingpositive active material layer and the upper positive active materiallayer may be different or may be the same (or partially the same).

As a positive active material in the upper positive active materiallayer, various positive active materials for preparing a lithium-ionsecondary battery in the art can be used. For example, the positiveactive material is a lithium-containing composite metal oxide, andparticularly for example, one or more of LiCoO₂, LiNiO₂, LiMn₂O₄,LiFePO₄, lithium nickel cobalt manganese oxide (such asLiNi_(0.8)Co_(0.01)Mn_(0.1)O₂) and lithium nickel manganese oxide.

The second polymer material (binder) in the upper positive activematerial layer may be, for example, a conventional binder such as PVDF,PVDC, SBR, CMC, polyacrylate, polycarbonate, polyethylene oxide or thelike.

The second conductive material in the upper positive active materiallayer may be, for example, a conventional conductive agent such as atleast one of conductive carbon black, acetylene black, graphite,graphene, carbon nanotubes, and carbon nanofibers.

(3) Current Collector

For the current collector, materials commonly used in the art can beused, preferably metal current collectors, for example metal flakes ormetal foils such as stainless steel, aluminum, copper, titanium or thelike. The metal current collector has a thickness of 4 μm to 16 μm.

Preferably, the current collector is a porous current collector (porousaluminum foil). The use of the porous aluminum foil can reduce theprobability of metal burrs occurrence and further reduce the probabilityof a severe aluminothermic reaction due to abnormal conditions such asnail penetration, thereby further improve the safety of the battery. Inaddition, the use of porous aluminum foil can also improve the wettingof the electrode plate by electrolyte, and thereby improve the dynamicperformance of the lithium-ion battery; while the underlying positiveactive material layer can cover the surface of the porous aluminum foilto prevent holidays of the upper active material layer during thecoating process.

In addition, considering the nail penetration safety, the elongation atbreak of the current collector has a great influence on the safety ofthe battery. If the elongation at break of the current collector is toolarge, the metal burr is large, which is not conducive to improving thesafety performance of the battery; if the elongation at break of thecurrent collector is too small, it is prone to breakage during theprocessing of the electrode plate compaction or when the battery issqueezed or collided, which will reduce the quality or safety of abattery. Therefore, in order to further improve the safety, particularlynail penetration safety, the elongation at break δ of the currentcollector should be no more than 4% and not less than 0.8%. Theelongation at break of the metal current collector can be adjusted bychanging the purity of the metal current collector, the impurity contentand the additive, the billet production process, the rolling speed, theheat treatment process, and the like.

The positive electrode plate of the battery according to thisapplication can be formed by a conventional method. For example, thefirst positive active material, the first polymer material, the firstconductive material, and optionally other auxiliary agents are dissolvedin a solvent and stirred to form a slurry, and then the slurry isapplied onto the current collector and heated, thus the underlyingpositive active material layer is obtained by drying. Then the secondpositive active material, the second polymer material, the secondconductive material, and optionally other auxiliary agents are dissolvedin a solvent and stirred to form a slurry, and then the slurry isapplied onto the underlying positive active material layer, and heated,thus the upper positive active material layer is obtained by drying.Then, the current collector containing the underlying positive activematerial layer and the upper positive active material layer is subjectedto post-treatment such as cold pressing, trimming, cutting and the liketo obtain a desired positive electrode plate.

Those skilled in the art will appreciate that various definition orpreferred ranges of the component selection, component content, andmaterial physicochemical properties (thickness, particle size, specificsurface area, elongation at break, etc.) in the various embodiments ofthe invention mentioned above can be combined arbitrarily. The combinedembodiments are still within the scope of the invention and areconsidered as part of the disclosure.

(4) Battery According to this Application

The battery according this application comprises a positive electrodeplate as described above, a separator, and a negative electrode plate.The negative electrode plate for use in conjunction with the positiveelectrode plate according to this application may be selected fromvarious conventional negative electrode plates in the art, and theconstitution and preparation method thereof are well known in the art.For example, the negative electrode plate may comprise a negativeelectrode current collector and a negative active material layerdisposed on the negative electrode current collector, and the negativeactive material layer may comprise a negative active material, a binder,a conductive material, and the like. The negative active material is,for example, a carbonaceous material such as graphite (artificialgraphite or natural graphite), conductive carbon black, carbon fiber, orthe like, a metal or a semimetal material such as Si, Sn, Ge, Bi, Sn,In, or an alloy thereof, and lithium-containing nitride orlithium-containing oxide, lithium metal or lithium aluminum alloy.

The separator used in the battery of this application may be selectedfrom various separators commonly used in the art.

The battery of the present invention typically also comprises anelectrolyte. Various electrolytes commonly used in the art, such assolutions of electrolytic salts in non-aqueous solvents, may be used.For example, for a lithium battery, a mixed solution of an electrolyticlithium salt and a non-aqueous solvent can be used. The electrolyticlithium salt may be selected from one or more of lithiumhexafluorophosphate (LiPF₆), lithium perchlorate, lithiumtetrafluoroborate, lithium hexafluoroarsenate, lithium halide, lithiumchloroaluminate, and lithium fluoroalkylsulfonate. The organic solventmay be selected from the group consisting of chain carbonates, cycliccarbonates, or a mixture thereof. The chain carbonate may be at leastone of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methylcarbonate (EMC), methyl propyl carbonate (MPC), dipropyl carbonate(DPC), and a chain-like organic ester containing fluorine, containingsulfur or having an unsaturated bond. The cyclic carbonate may be one ormore of ethylene carbonate (EC), propylene carbonate (PC), vinylenecarbonate (VC), γ-butyrolactone (γ-BL), sultone and other cyclic organicester containing fluorine, containing sulfur or having an unsaturatedbond.

The battery of this application may be a primary battery or a secondarybattery. The battery of this application may be a lithium-ion battery ora sodium-ion battery, preferably a lithium-ion battery, such as alithium-ion primary battery or a lithium-ion secondary battery. Inaddition to the use of the positive electrode plate as described above,the construction and preparation methods of these batteries are knownper se. Due to the use of the positive electrode plate as describedabove, the battery can have improved safety (e.g., nail penetrationsafety) and electrical properties. Furthermore, the positive electrodeplate according to this application can be easily processed, so that themanufacturing cost of the battery using the positive electrode plateaccording to this application can be reduced.

DETAILED DESCRIPTION

In order to make the objects, the technical solutions and the beneficialtechnical effects of this application more clear, this application willbe described in further detail below with reference to the embodiments.However, it is to be understood that the embodiments of this applicationare not intended to limit the invention, and the embodiments of theinvention are not limited to the embodiments set forth herein. Theexperimental conditions not indicated in the examples may refer toconventional conditions, or the conditions recommended by the materialsupplier or equipment supplier.

1. Preparation Method

The electrodes and batteries in the respective examples and comparativeexamples were prepared as follows unless otherwise specified.

1.1 Preparation of Positive Electrode Plate

1) Coating of the Underlying Active Material Layer

A certain ratio of a first polymer material, a first conductivematerial, a first positive active material (or inorganic filler) wasadded to N-methyl-2-pyrrolidone (NMP) to obtain a slurry, which wasuniformly stirred and then coated on both surfaces of the currentcollector, then the underlying positive active material layer wasobtained after drying at 85° C.

If it was necessary to crosslink the first polymer material, anactivator (sodium silicate) and a crosslinking agent were added afterthe slurry was obtained, and the mixture was uniformly stirred and thencoated on both surfaces of the current collector.

2) Coating of the Upper Active Material Layer

Then, 90 wt % of a second positive active material, 5 wt % of SP (secondconductive material), and 5 wt % of PVDF (second polymer material) weremixed with NMP as a solvent, uniformly stirred and then coated on theunderlying positive active material layer of the current collectorprepared according to the above method; then the upper positive activematerial layer was obtained after drying at 85° C.

3) Preparation of Electrode Plate

Then, the current collector with two layers of the positive activematerial was cold-pressed, then trimmed, cut, and stripped, and thendried under vacuum at 85° C. for 4 hours. After welding, the positiveelectrode plate meeting the requirements of the secondary battery wasobtained.

The main materials used in the specific examples were as follows:

Fluorinated polyolefin and/or chlorinated polyolefin polymer material:PVDF (Manufacturer “Solvay”, model 5130), PVDC (PVDF and PVDC materialsused in the examples, unless otherwise noted, are not crosslinked);

Difficultly soluble polymer materials: oil-dispersiblepolyacrylonitrile, oil-dispersible polyacrylic acid, water-dispersiblepolyacrylic acid, water-dispersible polyurethane, water-dispersiblepolyvinyl alcohol;

The first conductive material (conductive agent): Super-P (TIMCAL,Switzerland, abbreviated as SP);

The first positive active material: lithium iron phosphate (abbreviatedas LFP), carbon coating modified lithium iron phosphate (abbreviated asLFP/C), carbon coating modified lithium titanate (abbreviated asLi₄Ti₅O₁₂/C);

Inorganic filler: alumina;

Crosslinking agent: acrylonitrile, tetraisocyanate, polyethylene glycol;

The second positive active material: NCM811(LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂).

The above materials were commonly used materials in the lithium batteryindustry and could be commercially available from the correspondingsuppliers.

1.2 Preparation of Negative Electrode Plate

Negative electrode plate was prepared as follows: the active materialgraphite, conductive agent Super-P, thickener CMC, binder SBR were addedto the solvent deionized water at a mass ratio of 96.5:1.0:1.0:1.5 toform an anode slurry; then the slurry was coated on the surface of thenegative electrode current collector copper foil, and dried at 85° C.,then trimmed, cut, and stripped, and then dried under vacuum at 110° C.for 4 hours. After welding, the negative electrode plate meeting therequirements of the secondary battery was obtained.

1.3 Preparation of Electrolyte

Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethylcarbonate (DEC) were mixed at a volume ratio of 3:5:2 to obtain anEC/EMC/DEC mixed solvent, followed by dissolving the fully dried lithiumsalt LiPF₆ into a mixed organic solvent at a concentration of 1 mol/L toprepare an electrolyte.

1.4 Preparation of the Battery

A polypropylene film with a thickness of 12 μm was used as a separator,and the positive electrode, the separator and the negative electrodewere stacked in order, so that the separator was sandwiched in betweenthe positive electrode plate and the negative electrode plate, and thenthe stack was wound into a bare battery core. After vacuum baking at 75°C. for 10 h, the electrolyte (prepared as described in “1.4 Preparationof electrolyte” above) was injected therein followed by vacuum packageand standing for 24 h. After that, the battery core was charged to 4.2 Vwith a constant current of 0.1 C, and then was charged with a constantvoltage of 4.2 V until the current dropped to 0.05C, and then wasdischarged to 3.0V with a constant current of 0.1C. Above charging anddischarging processes were repeated twice. Finally, the battery core wascharged to 3.8V with a constant current of 0.1C, thereby completing thepreparation of the secondary battery.

2. Tests for Material Performances

In each of the examples and comparative examples, the physical propertyparameters of the materials were measured by a commonly known method inthe art, unless otherwise specified.

Some specific parameters were tested using the following methods.

2.1 Particle Size

The power sample was dispersed in a dispersion medium (distilled water),which was measured with a Malvern laser particle size analyzer MS2000for 5 times and averaged in unit of μm.

2.2 BET (Specific Surface Area)

The specific surface area of the powder sample of the test material wasmeasured with a Quadrasorb SI specific surface tester for 5 times andaveraged in unit of m²/g.

2.3 The Adhesion Force Between the Film Layer and the Current Collector

The electrode plate containing a current collector having a film layeron both sides was cut into a sample to be tested having a width of 2 cmand a length of 15 cm. One side of the sample to be tested was uniformlypasted on a stainless steel plate at 25° C. under normal pressure byusing 3M double-sided tape. One end of the sample to be tested was fixedon a GOTECH tensile machine, and the film layer of the sample to betested was stripped from the current collector by using the GOTECHtensile machine, wherein the maximum tensile force was read according tothe data diagram of the tensile force and the displacement. Theresulting value (in unit N) was divided by 0.02 to calculate theadhesion force (N/m).

2.4 Elongation at Break of Current Collector

Two samples having a length of 200 mm and a width of 15 mm were takenfrom the current collector. The sample was then mounted on a tensilemachine (model AI7000) and the average of the two tests was used as thetest result. Record the initial length L0, and start the tensilemachine, until the sample broke, and read the displacement L1 of thesample at the time of the break from the tensile machine. Elongation atbreak=(L1-L0)/L0 * 100%.

2.5 Thickness of the Current Collector, Thickness of the Coating andThickness of the Film Layer

Thickness of the current collector was measured by a micrometer, and theaverage value of 5 points was used.

Thickness of the coating and thickness of the film layer: first measurethe thickness of the current collector, and then measure the totalthickness after coating, and use the difference between the two valuesas the coating thickness. A similar method was used for the thickness ofthe film layer.

2.6 Cracking of the Coating

After drying and obtaining a positive active material layer, if nocracks were observed in the 100 m² electrode plate, it was defined as nocracking; if the number of occurrences of cracks in 100 m² electrodeplate was ≤3, it was defined as mild cracking; if the number ofoccurrences of cracks in 100 m² electrode plate was>3, it was defined assevere cracking.

2.7 Solubility of Polymer Materials in Oil Solvents

The polymer material was made into a film having a thickness of about 7μm, then cut into 20 mm*50 mm strips, weighed and recorded as M1;

The film was placed in NMP (N-methylpyrrolidone) solvent, placed at 130°C. for 5 min, taken out, and vacuum dried at 100° C.; after drying, itwas weighed and recorded as M2;

Then solubility was calculated as=(M1-M2)/M1 * 100%

In this application, for convenience of comparison, the solubility ofPVDF (manufacturer “Solvay”, model 5130) was used as a reference, and itwas recorded as 100%, and the ratio of the solubility of other materialsto the solubility of PVDF was recorded.

3. Test for Battery Performance

The safety performances of the secondary batteries from various examplesand comparative examples were evaluated using GBT31485-2015 “SafetyRequirements and Test Methods for Traction Battery of Electric Vehicle”,and the test results were recorded.

3.1 Puncture Test:

The secondary battery was fully charged to the charging cut-off voltagewith a current of 1 C, and then charged with a constant voltage untilthe current dropped to 0.05 C. After that, charging was terminated. Ahigh temperature resistant steel needle ofϕ5-10 mm (the tip thereof hada cone angle of 45°) was used to puncture the battery plate at a speedof 25 mm/s in the direction perpendicular to the battery plate. Thepuncture position should be close to the geometric center of the surfaceto be punctured, the steel needle stayed in the battery, and thenobserve if the battery had an indication of burning or exploding.

3.2 Overcharge Test:

The secondary battery was fully charged to the charging cut-off voltagewith a current of 1 C, and then charged with a constant voltage untilthe current dropped to 0.05 C. After that, charging was terminated.Then, after charging with a constant current of 1 C to reach 1.5 timesthe charging cut-off voltage or after charging for 1 hour, the chargingwas terminated.

3.3 Cycle Performance Test:

The test conditions of the cycle number were as follows: the secondarybattery was subjected to a 1C/1C cycle test at 25° C. in which thecharging and discharging voltage range was 2.8 to 4.2 V. The test wasterminated when the capacity was attenuated to 80% of the firstdischarging specific capacity.

3.4 PTC Effect Test

The secondary battery was fully charged to the charging cut-off voltagewith a current of 1 C, and then charged with a constant voltage untilthe current was reduced to 0.05 C. After that, the charging wasterminated and the DC resistance of the battery was tested (dischargingwith a current of 4 C for 10 s). Then, the battery core was placed at130° C. for 1 h followed by testing the DC resistance, and calculatingthe DC resistance growth rate. Then, the battery core was placed at 130°C. for 2 h followed by testing the DC resistance, and calculating the DCresistance growth rate.

3.5 DCR Test

The secondary battery was adjusted to 50% SOC with a current of 1 C at25° C., and the voltage U1 was recorded. Then, it was discharged with acurrent of 4 C for 30 seconds, and the voltage U2 was recorded.DCR=(U1-U2)/4C.

In this application, for convenience of comparison, the DCR of thebattery core in which the first polymer material contained onlynon-crosslinked PVDF as the polymer matrix was used as a reference, andwas recorded as 100%, and the DCR of the other battery cores and theratio thereof were calculated and recorded.

4. Performance Test Results

4.1 Effect of Underlying Active Material Layer on Nail PenetrationPerformance of the Battery

The corresponding positive electrode plate, negative electrode plate andbattery were prepared with the specific materials and amounts listed inTable 1-1 below according to the methods and procedures described in “1.Preparation method”, and were tested according to the method specifiedin “3. Tests for battery performance”. In order to ensure accuracy ofdata, 4 samples were prepared for each battery (10 samples for thepuncture test) and tested independently. The final test results wereaveraged and shown in Table 1-2 and Table 1-3.

In the test, the conventional electrode plate CPlate N was prepared withthe method described in “1.2 Preparation of negative electrode plate”;each positive electrode plate except the conventional electrode plateCPlate P, comparative electrode plate 1 and comparative electrode plate2, was prepared with the method described in “1.1 Preparation ofpositive electrode plate”, but the underlying positive active materiallayer was not provided, that is to say, only comprising the upperpositive active material layer; the comparative electrode plate 1(hereafter abbreviate as Comp. Plate 1) was basically prepared accordingto the method described in “1.1 Preparation of positive electrodeplate”, but the first positive active material was not added to theunderlying positive active material layer; the comparative electrodeplate 2 (hereafter abbreviate as Comp. Plate 2) was basically preparedaccording to the method described in “1.1 Preparation of positiveelectrode plate”, but the inorganic filler alumina was used to replacethe first positive active material.

TABLE 1-1 Composition of electrode plate Adhesion force Composition ofthe underlying positive active material layer between the The The firstThickness positive second The first The first positive active of theelectrode film positive polymer conductive polymer underlying layer andCurrent active material material material layer H current collectormaterial material wt % material wt % material wt % (μm) collector (N/m)CPlate P Al foil NCM811 / / / / / / / / Comp. Plate 1 Al foil NCM811PVDF 90 SP 10 / / 20 / Comp. Plate 2 Al foil NCM811 PVDC 35 SP 10Alumina 55 10 80 Plate 1-1 Al foil NCM811 PVDF 35 SP 10 LFP 55 3 100

TABLE 1-2 Performance of lithium-ion battery Positive Negative BatteryNo. electrode plate electrode plate Puncture Test Battery 1 CPlate PCPlate N 10 fail Battery 2 Comp. Plate 1 CPlate N 2 pass and 8 failBattery 3 Comp. Plate 2 CPlate N 10 pass Battery 4 Plate 1-1 CPlate N 10pass

TABLE 1-3 Performance of lithium-ion battery Positive Negative DCresistance DC resistance electrode electrode growth rate@ growth rate@Battery No. plate plate 130° C., 1 h 130° C., 2 h Battery 2 Comp. Plate1 CPlate N 20% 30% Battery 4 Plate 1-1 CPlate N 1200%  1500% 

The data in Table 1-1 and Table 1-2 indicated that when PVDF or PVDC wasused as the first polymer material (polymer matrix) and in the presenceof the first positive active material or other inorganic filler such asalumina, the adhesion force between the film layer and the currentcollector was greater than 10 N/m, which could greatly improve theneedle-puncture performance of the battery. This also indicated that theunderlying positive active material layer with higher PVDF or PVDCcontent could function as a binder layer. When an abnormal situationsuch as needle puncture or nailing occurred, metal burrs that might begenerated in the current collector could be wrapped, thereby effectivelypreventing the occurrence of internal short circuit of battery.

The data in Table 1-1 and Table 1-3 showed that when the underlyingpositive active material layer contained a higher content of PVDF andcontained a conductive material, the DC resistance increased when thetemperature increased, i.e. having a PTC effect; especially when theunderlying positive active material layer contained a relatively highcontent of PVDF and contained both a conductive material and a positiveactive material, the DC resistance growth rate was very remarkable. Thisindicated that when PVDF or PVDC was used as the first polymer material(polymer matrix) and in the presence of the first positive activematerial and the conductive material, the underlying positive activematerial layer had a remarkable PTC effect, which could improve thesafety performance of the battery significantly.

In summary, in the positive electrode plate of the battery according tothis application, the underlying positive active material layersimultaneously exerted the technical effects of the binder layer and thePTC safety coating, thereby greatly improving the nail-penetrationsafety performance of the battery.

4.2 Effect of the Content of Each Component Contained in the UnderlyingPositive Active Material Layer

Next, the effect of the content of each component contained in theunderlying positive active material layer will be further studied. Sincethe technical effects and laws of other inorganic fillers such asalumina and the first positive active material were substantially thesame in the “Puncture test”, in order to simplify the experiment,alumina was used instead of the first positive active material toillustrate the effect of each component on the underlying positiveactive material layer.

The corresponding positive electrode plate, negative electrode plate andbattery were prepared with the specific materials and amounts listed inTable 2-1 below according to the methods and procedures described in “1.Preparation method”, and then were tested according to the methodspecified in “3. Test for battery performance”. In order to ensure theaccuracy of data, 4 samples were prepared for each battery (10 samplesfor the puncture test) and tested independently. The final test resultswere averaged and shown in Table 2-2.

TABLE 2-1 Composition of electrode plate Adhesion force Composition ofthe underlying positive active material layer between the The Thicknesspositive second The first The first Other of the electrode film positivepolymer conductive inorganic underlying layer and Current activematerial material filler layer H current collector material material wt% material wt % material wt % (μm) collector (N/m) Comp. Plate 2-1 Alfoil NCM811 PVDF 75 SP 20 Alumina 5 8 160 Plate 2-2 Al foil NCM811 PVDF75 SP 15 Alumina 10 8 170 Plate 2-3 Al foil NCM811 PVDF 75 SP 10 Alumina15 8 180 Plate 2-4 Al foil NCM811 PVDF 60 SP 10 Alumina 30 8 135 Plate2-5 Al foil NCM811 PVDF 60 SP 8 Alumina 32 8 140 Plate 2-6 Al foilNCM811 PVDF 55 SP 15 Alumina 30 8 120 Plate 2-7 Al foil NCM811 PVDF 50SP 25 Alumina 25 8 110 Plate 2-8 Al foil NCM811 PVDF 40 SP 15 Alumina 458 95 Plate 2-9 Al foil NCM811 PVDF 35 SP 5 Alumina 60 8 75 Comp. Plate2-10 Al foil NCM811 PVDF 25 SP 5 Alumina 70 8 50 Comp. Plate 2-11 Alfoil NCM811 PVDF 5 SP 15 Alumina 80 8 8

TABLE 2-2 Performance of lithium-ion battery Positive Negative CycleLife Battery electrode electrode Puncture Test (cycle) Battery 6 Comp.Plate 2-1 CPlate N 5 fail, 5 pass 2502 Battery 7 Plate 2-2 CPlate N 10pass 2351 Battery 8 Plate 2-3 CPlate N 10 pass 2205 Battery 9 Plate 2-4CPlate N 10 pass 2251 Battery 10 Plate 2-5 CPlate N 10 pass 2000 Battery11 Plate 2-6 CPlate N 10 pass 2408 Battery 12 Plate 2-7 CPlate N 10 pass2707 Battery 13 Plate 2-8 CPlate N 10 pass 2355 Battery 14 Plate 2-9CPlate N 10 pass 1800 Battery 15 Comp. Plate 2-10 CPlate N 4 fail, 6pass 1715 Battery 16 Comp. Plate 2-11 CPlate N 9 fail, 1 passes 1750

The data in Table 2-1 and Table 2-2 demonstrated that: (1) if thecontent of other inorganic filler/first positive active material was toolow, the stability of the underlying positive active material layer wasnot sufficiently high, so the safety performance of the battery couldnot be fully improved; and if the content of other inorganicfiller/first positive active material was too high, the content of thefirst polymer material was too low and the effect thereof could not beexerted effectively, either; in addition, when the content of the firstpolymer material was too low, the adhesion force between the film layerand the current collector was less than 10 N/m, and the underlyingpositive active material layer could not effectively wrap the aluminumburr, thus the needle-puncture safety performance was greatlydeteriorated. (2) The conductive material had a great influence on theinternal resistance and polarization of the battery, thus affected thecycle life of the battery. The higher the content of the conductivematerial was, the smaller the internal resistance and polarization ofthe battery were, and the better the cycle life was.

After carrying out experiments, it was found that the suitable contentrange of each component of the safety coating was as follows: thecontent of the first polymer material was from 35 wt % to 75 wt %; thecontent of the first conductive material was from 5 wt % to 25 wt %; andthe content of other inorganic filler/first positive active material wasfrom 10 wt % to 60 wt %.

As long as the content of each component was within the above range, theeffect of improving the nail penetration safety and electricalproperties (e.g., cycle performance) of the battery can be achieved.

4.3 Effect of Inorganic Filler Types on Battery Performance

In order to further study the effect of material selection in theunderlying positive active material layer on the electrode plate and theperformance of the battery, the corresponding positive electrode plate,negative electrode plate and battery were prepared with the specificmaterials and amounts listed in Table 3-1 below according to the methodsand procedures described in “1. Preparation method”, and were testedaccording to the method specified in “3. Test for battery performance”.In order to ensure accuracy of data, 4 samples were prepared for eachbattery (10 samples for the puncture test) and tested independently. Thefinal test results were averaged which were shown in Table 3-2.

The data in Table 3-1 and Table 3-2 demonstrated that theelectrochemically active material (i.e., the first positive activematerial) could significantly improve the overcharge safety of thebattery compared with other inorganic fillers (such as alumina); inaddition, the carbon coated electrochemically active material alsoimproved the cycle life of the battery. Therefore, although otherinorganic fillers such as alumina and the electrochemically activematerial had similar effects on stabilizing the underlying positiveactive material layer and improving the nail penetration safety, whenusing the electrochemically active material (i.e., the first positiveactive material) to replace other inorganic fillers, the performance ofthe battery such as overcharge safety could be improved. Therefore itwas preferable to use the first positive active material in theunderlying positive active material layer, in particular lithium ironphosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithiummanganese phosphate, and lithium manganese iron phosphate, etc werepreferred.

TABLE 3-1 Composition of electrode plate Adhesion force Composition ofthe underlying positive active material layer between the The Thicknesspositive second The first The first Other inorganic filler of theelectrode film positive polymer conductive Carbon underlying layer andCurrent active material material content layer H current collectormaterial material wt % material wt % material wt % (wt %) (μm) collectorPlate 2-41 Al foil NCM811 PVDF 60 SP 10 alumina 30 / 8 135 Plate 2-42 Alfoil NCM811 PVDF 60 SP 10 LFP 30 / 8 155 Plate 2-43 Al foil NCM811 PVDF60 SP 10 LFP/C 30 1 8 155 Plate 2-44 Al foil NCM811 PVDF 60 SP 10 LFP/C30 2 8 150 Plate 2-45 Al foil NCM811 PVDF 60 SP 10 LFP/C 30 3 8 147Plate 2-46 Al foil NCM811 PVDF 60 SP 10 Li₄Ti₅O₁₂/C 30 5 8 143

TABLE 3-2 Performance of lithium-ion battery Positive Negative PunctureOvercharge Cycle test Battery electrode electrode Test test (cycle)Battery 46 Plate 2-41 CPlate N 10 pass 10 fail 2200 Battery 47 Plate2-42 CPlate N 10 pass 10 pass 2300 Battery 48 Plate 2-43 CPlate N 10pass 10 pass 2500 Battery 49 Plate 2-44 CPlate N 10 pass 10 pass 2700Battery 50 Plate 2-45 CPlate N 10 pass 10 pass 2900 Battery 51 Plate2-46 CPlate N 10 pass 10 pass 3000

4.4 Effect of Crosslinking on Battery Performance

The corresponding positive electrode plate, negative electrode plate andbattery were prepared with the specific materials and amounts listed inTable 4-1 below according to the methods and procedures described above,and were tested according to the specified method to study the effect ofthe crosslinking of the first polymer material on coating cracking andDCR, the results were shown in Table 4-2.

In the case where the coating speed of the upper positive activematerial layer was 50 m/min, for the electrode plate 2-51, the firstpolymer material was not crosslinked by adding a crosslinking agent, andthe electrode plate was severely cracked. The addition of a crosslinkingagent had a significant effect on improving the cracking of theelectrode plate. No cracking occurred in the electrode plate 2-53 to theelectrode plate 2-56. Similar experiments were performed for PVDC(electrode plates 2-57 and 2-58) and the results were similar. It can beseen that the addition of the crosslinking agent can significantlyeliminate the coating cracking of the electrode plate.

For the electrode plate 2-51, the first polymer material was notcrosslinked by adding a crosslinking agent, and the first polymermaterial swelled greatly in the electrolyte, resulting in a large DCR.The addition of the crosslinking agent can reduce the swelling of thefirst polymer material in the electrolyte, and had a significant effecton reducing DCR. It can be seen that the addition of the crosslinkingagent can significantly reduce the DCR of the battery.

In addition, the above data indicated that PVDF/PVDC can be used as thefirst polymer material regardless of crosslinking, and the underlyingpositive active material layer had a PTC effect, and the obtainedbattery had high safety (excellent test results of puncture test). Itindicated that the crosslinking treatment did not adversely affect theprotective effect of the underlying positive active material layer.Furthermore, compared with the uncrosslinked PVDC/PVDF, the crosslinkingtreatment improved the cracking of the electrode plate, from severecracking to no cracking or mild cracking. The crosslinking treatment canreduce the swelling of the first polymer material in the electrolyte,thereby reducing the DCR by 15% to 25%, thereby improving the electricalproperties of the battery.

TABLE 4-1 Effect of crosslinking agent Composition of the underlyingpositive active material layer The Crosslinking agent Thickness secondThe first The first Ratio of the Cracking positive The first polymerconductive positive active to the underlying (coating Current activematerial material material polymer layer H speed collector materialmaterial wt % material wt % material wt % type material (μm) 50 m/min)Plate 2-51 Al foil NCM811 Uncrosslinked 60 SP 10 LFP/C 30 No 0 8 SeverePVDF cracking Plate 2-52 Al foil NCM811 Crosslinked 60 SP 10 LFP/C 30Acrylonitrile 0.01%  8 Mild PVDF cracking Plate 2-53 Al foil NCM811Crosslinked 60 SP 10 LFP/C 30 Tetraisocyanate 0.1% 8 No PVDF crackingPlate 2-54 Al foil NCM811 Crosslinked 60 SP 10 LFP/C 30 Polyethylene0.5% 8 No PVDF glycol cracking Plate 2-55 Al foil NCM811 Crosslinked 60SP 10 LFP/C 30 Acrylonitrile 1.5% 8 No PVDF cracking Plate 2-56 Al foilNCM811 Crosslinked 60 SP 10 LFP/C 30 Acrylonitrile   5% 8 No PVDFcracking Plate 2-57 Al foil NCM811 Uncrosslinked 60 SP 10 LFP/C 30 No 08 Severe PVDC cracking Plate 2-58 Al foil NCM811 Crosslinked 60 SP 10LFP/C 30 Acrylonitrile   3% 8 No PVDC cracking

TABLE 4-2 Performance of lithium-ion battery Positive Negative DCR ofthe Battery electrode electrode battery Puncture Test Battery 52 Plate2-51 CPlate N 100%  10 pass Battery 53 Plate 2-52 CPlate N 80% 10 passBattery 54 Plate 2-53 CPlate N 85% 10 pass Battery 55 Plate 2-54 CPlateN 78% 10 pass Battery 56 Plate 2-55 CPlate N 75% 10 pass Battery 57Plate 2-56 CPlate N 84% 10 pass

4.5 Effect of the First Polymer Material Type on Battery Performance

The corresponding positive electrode plate, negative electrode plate andbattery were prepared with the specific materials and amounts listed inTable 5-1 below according to the methods and procedures described above,and were tested according to the specified method to study the effect ofthe composition of the first polymer material on the cracking of thecoating, etc., and the results were shown in Table 5-2.

The data in Table 5-1 and Table 5-2 showed that when the underlyingpositive active material layer did not contain a difficultly solublepolymer material (i.e. a polymer material having a solubility in an oilsolvent smaller than that of a fluorinated polyolefin), the electrodeplate was prone to cracking. After the addition of the difficultlysoluble polymer material, no cracking occurred. This was because thatthe difficultly soluble polymer material used as the “difficultlysoluble component” greatly reduced the dissolution and swelling of thePVDF-based polymer material contained in the underlying positive activematerial layer caused by the organic oil solvent in the upper activematerial slurry, thereby reducing cracking and significantly increasingproduction efficiency. Moreover, after the addition of the difficultlysoluble polymer material, the battery's DCR was significantly reduced.It can be seen that the introduction of difficultly soluble polymermaterials can significantly reduce the DCR of the battery.

Moreover, in the case where a difficultly soluble polymer material wasadded, the result of the puncture test was still greatly improved ascompared with the conventional positive electrode plate. This indicatedthat when the combination of PVDC and/or PVDF-based polymer materialsand difficultly soluble polymer materials was used as the first polymermaterial, the battery's safety performance was still excellent, and thebattery's DCR was improved, and the quality stability and productionefficiency of the electrode plate was improved, so the overallperformance of the battery was better.

TABLE 5-1 Composition of electrode plate Composition of the underlyingpositive active material layer The The first polymer material The firstpositive active Thickness Polarity second difficultly material of the ofthe positive soluble The first conductive Carbon underlying electrodeactive Fluorinated (wt %) polymer material content layer H platematerial polyolefin (B1) material (wt %) material (wt %) material (wt %)(wt %) (μm) Plate 2-61 Positive NCM811 PVDF 60 / / SP 10 LFP/C 30 1 8Plate 2-62 Positive NCM811 PVDF 50 Water- 10 SP 10 LFP/C 30 1 8dispersible polyacrylic acid Plate 2-63 Positive NCM811 PVDF 40 Water-20 SP 10 LFP/C 30 1 8 dispersible polyurethane Plate 2-64 PositiveNCM811 PVDF 30 Water- 30 SP 10 LFP/C 30 1 8 dispersible polyvinylalcohol Plate 2-65 Positive NCM811 PVDF 40 Oil-dispersible 20 SP 10LFP/C 30 1 8 polyacrylate Plate 2-66 Positive NCM811 PVDF 30Oil-dispersible 30 SP 10 LFP/C 30 1 8 polyacrylate Plate 2-67 PositiveNCM811 PVDF 25 Oil-dispersible 35 SP 10 LFP/C 30 1 8 polyacrylic acidPlate 2-68 Positive NCM811 PVDF 20 Oil-dispersible 40 SP 10 LFP/C 30 1 8polyacrylic acid

TABLE 5-2 Performance of lithium-ion battery Cracking (coating DCRPositive Negative speed 50 of the Battery electrode electrode PunctureTest m/min) battery Battery 61 Plate 2-61 CPlate N 10 pass Severe 100% cracking Battery 62 Plate 2-62 CPlate N 9 pass, 1 fails No cracking 55%Battery 63 Plate 2-63 CPlate N 8 pass, 2 fail No cracking 50% Battery 64Plate 2-64 CPlate N 7 pass, 3 fail No cracking 40% Battery 65 Plate 2-65CPlate N 10 pass No cracking 90% Battery 66 Plate 2-66 CPlate N 10 passNo cracking 70% Battery 67 Plate 2-67 CPlate N 10 pass No cracking 64%Battery 68 Plate 2-68 CPlate N 10 pass No cracking 60%

It will be understood by those skilled in the art that the aboveapplication examples of the electrode plate of this application are onlyexemplified to be used for a lithium battery, but the electrode plate ofthis application can also be applied to other types of batteries orelectrochemical devices, and still may produce good technical effect ofthis application.

It will be apparent to those skilled in the art that the presentapplication may be modified and varied in accordance with the aboveteachings. Accordingly, the present application is not limited to thespecific embodiments disclosed and described above, and modificationsand variations of the present application are intended to be includedwithin the scope of the claims of the present application. In addition,although some specific terminology is used in this specification, theseterms are for convenience of illustration only and are not intended tolimit the present application in any way.

The invention claimed is:
 1. A battery comprising a positive electrodeplate, a separator, and a negative electrode plate, wherein the positiveelectrode plate comprises a positive electrode current collector and atleast two layers of positive active material coated on at least onesurface of the positive electrode current collector, comprising anunderlying positive active material layer in contact with the positiveelectrode current collector and an upper positive active material layerin contact with the underlying positive active material layer and awayfrom the positive electrode current collector; and the at least twolayers of positive active material layers are collectively referred toas a positive electrode film layer; and wherein the underlying positiveactive material layer comprises a first positive active material, afirst polymer material and a first conductive material, and based on thetotal weight of the underlying layer positive active material layer, thefirst positive active material has a content of A % by weight, the firstpolymer material has a content of B % by weight, and the firstconductive material has a content of C % by weight; and wherein theupper positive active material layer comprises a second positive activematerial, a second polymer material and a second conductive material,and based on the total weight of the upper positive active materiallayer, the second positive active material has a content of A′ % byweight, the second polymer material has a content of B′ % by weight, andthe second conductive material has a content of C′% by weight; wherein A%<A′ %, B %>B′ %, C %≥C′ %; and wherein an adhesion force between thepositive electrode film layer and the positive electrode currentcollector is at least 10 N/m; wherein the first polymer material is amixed material of fluorinated polyolefin and/or chlorinated polyolefinpolymer material having a crosslinked structure with a difficultlysoluble polymer material, wherein the difficultly soluble polymermaterial has a solubility in an oil solvent, which is less than thesolubility of the fluorinated polyolefin and/or chlorinated polyolefinpolymer material in the oil solvent; wherein based on the total weightof the underlying positive active material layer, the fluorinatedpolyolefin and/or chlorinated polyolefin polymer material has a contentof B1% satisfying B1%≥17.5 wt %.
 2. The battery according to claim 1,wherein the first positive active material satisfies 10 wt %≤A %≤60 wt%, the first polymer material satisfies 35 wt %≤B %≤75 wt %, and thefirst conductive material satisfies 5 wt %≤C %≤25 wt %; and wherein thesecond positive active material satisfies 90 wt % A′ % 99 wt %, thesecond polymer material satisfies 0.5 wt %≤B′ %≤5 wt %, and the secondconductive material satisfies 0.5 wt % ≤C′ %≤5 wt %.
 3. The batteryaccording to claim 1, wherein the oil solvent is NMP.
 4. The batteryaccording to claim 1, wherein the difficultly soluble polymer materialis an oil-dispersible polymer material or a water-dispersible polymermaterial; and wherein the oil-dispersible polymer material is selectedfrom at least one of oil-dispersible polyacrylonitrile, oil-dispersiblepolyacrylic acid, oil-dispersible polyacrylate, oil-dispersiblepolyacrylic acid-acrylate, oil-dispersible polyacrylonitrile-acrylicacid, and oil-dispersible polyacrylonitrile-acrylate; and wherein thewater-dispersible polymer material is selected from at least one ofwater-dispersible polyacrylic acid, water-dispersible polyurethane,water-dispersible polyvinyl alcohol, water-dispersible PVDF,water-dispersible polyacrylate, water-dispersiblepolytetrafluoroethylene, and water-dispersible polyacrylonitrile.
 5. Thebattery according to claim 1, wherein the fluorinated polyolefin and/orchlorinated polyolefin polymer material is selected from at least one ofpolyvinylidene fluoride (PVDF), carboxylic acid modified PVDF, acrylicacid modified PVDF, polyvinylidene chloride (PVDC), carboxylic acidmodified PVDC, acrylic acid modified PVDC, PVDF copolymer and PVDCcopolymer.
 6. The battery according to claim 1, wherein the firstconductive material is selected from at least one of a conductivecarbon-based material, a conductive metal material, and a conductivepolymer material; and wherein the conductive carbon-based material isselected from at least one of conductive carbon black, acetylene black,graphite, graphene, carbon nanotubes, and carbon nanofibers; and whereinthe conductive metal material is selected from at least one of Alpowder, Ni powder, and gold powder.
 7. The battery according to claim 1,wherein the first positive active material is selected from at least oneof lithium cobalt oxide, lithium nickel manganese cobalt oxide, lithiumnickel manganese aluminate, lithium iron phosphate, lithium vanadiumphosphate, lithium cobalt phosphate, lithium manganese phosphate,lithium iron manganese phosphate, lithium iron silicate, lithiumvanadium silicate, lithium cobalt silicate, lithium manganese silicate,spinel lithium manganese oxide, spinel lithium nickel manganese oxide,lithium titanium oxide, or at least one of a conductive carbon coatingmodified above material, a conductive metal coating modified abovematerial or a conductive polymer coating modified above material.
 8. Thebattery according to claim 1, wherein the first positive active materialis at least one of lithium iron phosphate, lithium vanadium phosphate,lithium cobalt phosphate, lithium manganese phosphate, lithium manganeseiron phosphate or at least one of a conductive carbon coating modifiedabove material, a conductive metal coating modified above material or aconductive polymer coating modified above material.
 9. The batteryaccording to claim 1, wherein the first positive active material has aspecific surface area (BET) of at most 500 m²/g.
 10. The batteryaccording to claim 1, wherein the positive electrode current collectoris a metal current collector, and wherein the metal current collectorhas a thickness of 4 μm to 16 μm; and wherein the metal currentcollector has an elongation at break δ satisfying 0.8%≤δ≤64%.
 11. Thebattery according to claim 1, wherein the battery has a DC resistancegrowth rate of 100% or more at 130° C.